The objective of this project is to investigate the involvement of coherent amplitude modulation (AM) in sound source determination. Due to the nature of many sound sources, the spectral components that originate from a single source often share a common or coherent AM. The amplitudes of the components of a single sound source vary in a similar manner over time. The aim of the project is to investigate the role that this coherent modulation may play in the fusion and segregation of auditory images as a fundamental ingredient in sound source determination. This proposal has three parts: The major effort will be directed toward human psychophysics. However, we want to move toward a multidisciplinary study of modulation processing. Therefore, we describe work involving animal psychophysics and auditory physiology at the level of the auditory nerve and cochlear nucleus. Three objectives are to be pursued in the human psychophysical experiments: the first objective is concerned with the manner in which modulation information from specific audio-frequency regions can be combined, the second objective evaluates selectivity with masking and discrimination procedures that require listeners to process envelope coherence among specific spectral components in a complex sound consisting of many different spectral components, and the third objective is to model auditory envelope detection to allow for selective cross-spectral processing. The effects measured on human listeners in the past [most notably Modulation Detection Interference (MDI)] and many of those we plan to measure during this grant period will be investigated in a mammalian animal model, the Chinchilla. The overall aim of the physiology section of the project is to understand amplitude modulation processing primarily at the level of the cochlear nucleus in the Chinchilla, especially for complex, low-rate modulation patterns. As in the other projects, this three prong approach is crucial if one is to understand auditory processing of complex sounds.